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An Introduction to MPI Parallel Programming with the Message Passing Interface

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Title: An Introduction to MPI Parallel Programming with the Message Passing Interface


1
An Introduction to MPIParallel Programming with
the Message Passing Interface
  • William Gropp
  • Ewing Lusk
  • Argonne National Laboratory
  • Presenter Mike Slavik

2
Outline
  • Background
  • The message-passing model
  • Origins of MPI and current status
  • Sources of further MPI information
  • Basics of MPI message passing
  • Hello, World!
  • Fundamental concepts
  • Simple examples in Fortran and C
  • Extended point-to-point operations
  • non-blocking communication
  • modes

3
Outline (continued)
  • Advanced MPI topics
  • Collective operations
  • More on MPI datatypes
  • Application topologies
  • The profiling interface
  • Toward a portable MPI environment

4
Companion Material
  • Online examples available athttp//www.mcs.anl.go
    v/mpi/tutorials/perf
  • ftp//ftp.mcs.anl.gov/mpi/mpiexmpl.tar.gz
    contains source code and run scripts that allows
    you to evaluate your own MPI implementation

5
The Message-Passing Model
  • A process is (traditionally) a program counter
    and address space.
  • Processes may have multiple threads (program
    counters and associated stacks) sharing a single
    address space. MPI is for communication among
    processes, which have separate address spaces.
  • Interprocess communication consists of
  • Synchronization
  • Movement of data from one processs address space
    to anothers.

6
Types of Parallel Computing Models
  • Data Parallel - the same instructions are carried
    out simultaneously on multiple data items (SIMD)
  • Task Parallel - different instructions on
    different data (MIMD)
  • SPMD (single program, multiple data) not
    synchronized at individual operation level
  • SPMD is equivalent to MIMD since each MIMD
    program can be made SPMD (similarly for SIMD, but
    not in practical sense.)

Message passing (and MPI) is for MIMD/SPMD
parallelism. HPF is an example of an SIMD
interface.
7
Cooperative Operations for Communication
  • The message-passing approach makes the exchange
    of data cooperative.
  • Data is explicitly sent by one process and
    received by another.
  • An advantage is that any change in the receiving
    processs memory is made with the receivers
    explicit participation.
  • Communication and synchronization are combined.

8
One-Sided Operations for Communication
  • One-sided operations between processes include
    remote memory reads and writes
  • Only one process needs to explicitly participate.
  • An advantage is that communication and
    synchronization are decoupled
  • One-sided operations are part of MPI-2.

Process 0
Process 1
(memory)
Get(data)
9
What is MPI?
  • A message-passing library specification
  • extended message-passing model
  • not a language or compiler specification
  • not a specific implementation or product
  • For parallel computers, clusters, and
    heterogeneous networks
  • Full-featured
  • Designed to provide access to advanced parallel
    hardware for
  • end users
  • library writers
  • tool developers

10
MPI Sources
  • The Standard itself
  • at http//www.mpi-forum.org
  • All MPI official releases, in both postscript and
    HTML
  • Books
  • Using MPI Portable Parallel Programming with
    the Message-Passing Interface, by Gropp, Lusk,
    and Skjellum, MIT Press, 1994.
  • MPI The Complete Reference, by Snir, Otto,
    Huss-Lederman, Walker, and Dongarra, MIT Press,
    1996.
  • Designing and Building Parallel Programs, by Ian
    Foster, Addison-Wesley, 1995.
  • Parallel Programming with MPI, by Peter Pacheco,
    Morgan-Kaufmann, 1997.
  • MPI The Complete Reference Vol 1 and 2,MIT
    Press, 1998(Fall).
  • Other information on Web
  • at http//www.mcs.anl.gov/mpi
  • pointers to lots of stuff, including other talks
    and tutorials, a FAQ, other MPI pages

11
Why Use MPI?
  • MPI provides a powerful, efficient, and portable
    way to express parallel programs
  • MPI was explicitly designed to enable libraries
  • which may eliminate the need for many users to
    learn (much of) MPI

12
A Minimal MPI Program (C)
  • include "mpi.h"
  • include ltstdio.hgt
  • int main( int argc, char argv )
  • MPI_Init( argc, argv )
  • printf( "Hello, world!\n" )
  • MPI_Finalize()
  • return 0

13
Notes on C and Fortran
  • C and Fortran bindings correspond closely
  • In C
  • mpi.h must be included
  • MPI functions return error codes or MPI_SUCCESS
  • In Fortran
  • mpif.h must be included, or use MPI module
    (MPI-2)
  • All MPI calls are to subroutines, with a place
    for the return code in the last argument.
  • C bindings, and Fortran-90 issues, are part of
    MPI-2.

14
Error Handling
  • By default, an error causes all processes to
    abort.
  • The user can cause routines to return (with an
    error code) instead.
  • In C, exceptions are thrown (MPI-2)
  • A user can also write and install custom error
    handlers.
  • Libraries might want to handle errors differently
    from applications.

15
Running MPI Programs
  • The MPI-1 Standard does not specify how to run an
    MPI program, just as the Fortran standard does
    not specify how to run a Fortran program.
  • In general, starting an MPI program is dependent
    on the implementation of MPI you are using, and
    might require various scripts, program arguments,
    and/or environment variables.
  • mpiexec ltargsgt is part of MPI-2, as a
    recommendation, but not a requirement
  • You can use mpiexec for MPICH and mpirun for
    SGIs MPI in this class

16
Finding Out About the Environment
  • Two important questions that arise early in a
    parallel program are
  • How many processes are participating in this
    computation?
  • Which one am I?
  • MPI provides functions to answer these questions
  • MPI_Comm_size reports the number of processes.
  • MPI_Comm_rank reports the rank, a number between
    0 and size-1, identifying the calling process

17
Better Hello (C)
  • include "mpi.h"
  • include ltstdio.hgt
  • int main( int argc, char argv )
  • int rank, size
  • MPI_Init( argc, argv )
  • MPI_Comm_rank( MPI_COMM_WORLD, rank )
  • MPI_Comm_size( MPI_COMM_WORLD, size )
  • printf( "I am d of d\n", rank, size )
  • MPI_Finalize()
  • return 0

18
MPI Basic Send/Receive
  • We need to fill in the details in
  • Things that need specifying
  • How will data be described?
  • How will processes be identified?
  • How will the receiver recognize/screen messages?
  • What will it mean for these operations to
    complete?

19
What is message passing?
  • Data transfer plus synchronization

Process 0
Process 1
Time
  • Requires cooperation of sender and receiver
  • Cooperation not always apparent in code

20
Some Basic Concepts
  • Processes can be collected into groups.
  • Each message is sent in a context, and must be
    received in the same context.
  • A group and context together form a communicator.
  • A process is identified by its rank in the group
    associated with a communicator.
  • There is a default communicator whose group
    contains all initial processes, called
    MPI_COMM_WORLD.

21
MPI Datatypes
  • The data in a message to sent or received is
    described by a triple (address, count, datatype),
    where
  • An MPI datatype is recursively defined as
  • predefined, corresponding to a data type from the
    language (e.g., MPI_INT, MPI_DOUBLE_PRECISION)
  • a contiguous array of MPI datatypes
  • a strided block of datatypes
  • an indexed array of blocks of datatypes
  • an arbitrary structure of datatypes
  • There are MPI functions to construct custom
    datatypes, such an array of (int, float) pairs,
    or a row of a matrix stored columnwise.

22
MPI Tags
  • Messages are sent with an accompanying
    user-defined integer tag, to assist the receiving
    process in identifying the message.
  • Messages can be screened at the receiving end by
    specifying a specific tag, or not screened by
    specifying MPI_ANY_TAG as the tag in a receive.
  • Some non-MPI message-passing systems have called
    tags message types. MPI calls them tags to
    avoid confusion with datatypes.

23
MPI Basic (Blocking) Send
  • MPI_SEND (start, count, datatype, dest, tag,
    comm)
  • The message buffer is described by (start, count,
    datatype).
  • The target process is specified by dest, which is
    the rank of the target process in the
    communicator specified by comm.
  • When this function returns, the data has been
    delivered to the system and the buffer can be
    reused. The message may not have been received
    by the target process.

24
MPI Basic (Blocking) Receive
  • MPI_RECV(start, count, datatype, source, tag,
    comm, status)
  • Waits until a matching (on source and tag)
    message is received from the system, and the
    buffer can be used.
  • source is rank in communicator specified by comm,
    or MPI_ANY_SOURCE.
  • status contains further information
  • Receiving fewer than count occurrences of
    datatype is OK, but receiving more is an error.

25
Retrieving Further Information
  • Status is a data structure allocated in the
    users program.
  • In C
  • int recvd_tag, recvd_from, recvd_count
  • MPI_Status status
  • MPI_Recv(..., MPI_ANY_SOURCE, MPI_ANY_TAG, ...,
    status )
  • recvd_tag status.MPI_TAG
  • recvd_from status.MPI_SOURCE
  • MPI_Get_count( status, datatype, recvd_count )

26
Why Datatypes?
  • Since all data is labeled by type, an MPI
    implementation can support communication between
    processes on machines with very different memory
    representations and lengths of elementary
    datatypes (heterogeneous communication).
  • Specifying application-oriented layout of data in
    memory
  • reduces memory-to-memory copies in the
    implementation
  • allows the use of special hardware
    (scatter/gather) when available

27
Tags and Contexts
  • Separation of messages used to be accomplished by
    use of tags, but
  • this requires libraries to be aware of tags used
    by other libraries.
  • this can be defeated by use of wild card tags.
  • Contexts are different from tags
  • no wild cards allowed
  • allocated dynamically by the system when a
    library sets up a communicator for its own use.
  • User-defined tags still provided in MPI for user
    convenience in organizing application
  • Use MPI_Comm_split to create new communicators

28
MPI is Simple
  • Many parallel programs can be written using just
    these six functions, only two of which are
    non-trivial
  • MPI_INIT
  • MPI_FINALIZE
  • MPI_COMM_SIZE
  • MPI_COMM_RANK
  • MPI_SEND
  • MPI_RECV
  • Point-to-point (send/recv) isnt the only way...

29
Introduction to Collective Operations in MPI
  • Collective operations are called by all processes
    in a communicator.
  • MPI_BCAST distributes data from one process (the
    root) to all others in a communicator.
  • MPI_REDUCE combines data from all processes in
    communicator and returns it to one process.
  • In many numerical algorithms, SEND/RECEIVE can be
    replaced by BCAST/REDUCE, improving both
    simplicity and efficiency.

30
Example PI in C -1
  • include "mpi.h"
  • include ltmath.hgt
  • int main(int argc, char argv)
  • int n, myid, numprocs, i, rcdouble PI25DT
    3.141592653589793238462643double mypi, pi, h,
    sum, x, aMPI_Init(argc,argv)MPI_Comm_size(MP
    I_COMM_WORLD,numprocs)MPI_Comm_rank(MPI_COMM_WO
    RLD,myid)for() if (myid 0)
    printf("Enter the number of intervals (0 quits)
    ") scanf("d",n) MPI_Bcast(n, 1,
    MPI_INT, 0, MPI_COMM_WORLD) if (n 0) break

31
Example PI in C - 2
  • h 1.0 / (double) n sum 0.0 for (i
    myid 1 i lt n i numprocs) x h
    ((double)i - 0.5) sum 4.0 / (1.0 xx)
    mypi h sum MPI_Reduce(mypi, pi, 1,
    MPI_DOUBLE, MPI_SUM, 0,
    MPI_COMM_WORLD) if (myid 0) printf("pi
    is approximately .16f, Error is .16f\n",
    pi, fabs(pi - PI25DT))MPI_Finalize()
  • return 0

32
Alternative set of 6 Functions for Simplified MPI
  • MPI_INIT
  • MPI_FINALIZE
  • MPI_COMM_SIZE
  • MPI_COMM_RANK
  • MPI_BCAST
  • MPI_REDUCE
  • What else is needed (and why)?

33
Sources of Deadlocks
  • Send a large message from process 0 to process 1
  • If there is insufficient storage at the
    destination, the send must wait for the user to
    provide the memory space (through a receive)
  • What happens with
  • This is called unsafe because it depends on the
    availability of system buffers

34
Some Solutions to the unsafe Problem
  • Order the operations more carefully
  • Use non-blocking operations

35
Toward a Portable MPI Environment
  • MPICH is a high-performance portable
    implementation of MPI (1).
  • It runs on MPP's, clusters, and heterogeneous
    networks of workstations.
  • In a wide variety of environments, one can do
  • configure
  • make
  • mpicc -mpitrace myprog.c
  • mpirun -np 10 myprog
  • upshot myprog.log
  • to build, compile, run, and analyze performance.

36
Some Simple Exercises
  • Compile and run the hello and pi programs.
  • Modify the pi program to use send/receive instead
    of bcast/reduce.
  • Write a program that sends a message around a
    ring. That is, process 0 reads a line from the
    terminal and sends it to process 1, who sends it
    to process 2, etc. The last process sends it
    back to process 0, who prints it.
  • Time programs with MPI_WTIME. (Find it.)

37
When to use MPI
  • Portability and Performance
  • Irregular Data Structures
  • Building Tools for Others
  • Libraries
  • Need to Manage memory on a per processor basis

38
When not to use MPI
  • Regular computation matches HPF
  • But see PETSc/HPF comparison (ICASE 97-72)
  • Solution (e.g., library) already exists
  • http//www.mcs.anl.gov/mpi/libraries.html
  • Require Fault Tolerance
  • Sockets
  • Distributed Computing
  • CORBA, DCOM, etc.

39
Summary
  • The parallel computing community has cooperated
    on the development of a standard for
    message-passing libraries.
  • There are many implementations, on nearly all
    platforms.
  • MPI subsets are easy to learn and use.
  • Lots of MPI material is available.
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